U.S. President Barack Obama delivers the eulogy for South Carolina state senator and Rev. Clementa Pinckney during Pinckney's funeral service June 26, 2015 in Charleston, South Carolina. Win McNamee/Getty Images In one of the most poignant moments of his presidency, President Barack Obama abruptly began singing "Amazing Grace" during a eulogy for the Rev. Clementa Pinckney, one of nine people who died in a shooting last month at an historically African-American church in Charleston, South Carolina.

Valerie Jarrett, a senior adviser to Obama, told the backstory of that moment during a recent conversation at the Aspen Institute's Ideas Festival, calling it a "powerful moment that I will never forget."

Jarrett said the idea was Obama's — but it earned a mixed reception when he first proposed it to her and First Lady Michelle Obama aboard Marine One en route to South Carolina for the eulogy.

"He looked at me like, 'Don't say, don't sing,'" Jarrett recalled Obama saying. She said she had urged him not to sing a rendition of Al Green's, "Let's Stay Together," at an event in 2012.

"Hmm," Jarrett said she replied upon hearing Obama's proposal.

The First Lady, however, was more outwardly skeptical.

"Why on earth would that fit in?" she asked, according to Jarrett.

Obama went on to explain his reasoning, ultimately saying that he wasn't sure he was going to sing — he just wanted to warn them of the possibility.

"We'll see how it feels at the time," Obama told them.

Jarrett said it was clear that from the time he began his eulogy, the church was "clearly with him."

"There was a lot of, 'Yeah, you take your time, Mr. President," and, 'Yeah, that's right,' and, 'Slow down,' and, 'I feel ya,' and all the kinds of things you get in a black church," Jarrett said. "And so, he knew they were with him."

After the fact, Jarrett asked Obama about the brief pause before he began singing. She asked him if he was still questioning whether to do it.

"Oh no, I knew I was going to sing," Obama told her. "I was just trying to figure out which key I was going to sing in."

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